Reactivity of some condensed heterocyclic cations towards ortho-positronium atoms in acetonitrile solution

The effect of changes in chemical structure and charge distribution of molecules on their reactivity towards an orthopositronium (o-Ps) atom was investigated by positron annihilation lifetime spectroscopy. Molecules with very similar structural frameworks were selected as model systems. Bridgehead nitrogen-containing condensed heterocyclic pyridoazinium salts with 1-3 nitrogen atoms and their methyl-substituted analogues were studied in acetonitrile solutions. In both groups the reactivity towards o-Ps atoms increased drastically with the increasing number of N atoms, while only a slight decrease of reactivity was observed due to methyl substitution. The strong electron-withdrawing effect of the N atoms and the weak electron-donating character of the methyl group accounted for these observations. In contrast to the reaction with o-Ps atom, methyl substitution affected drastically the chemical reactivity of these molecules in ring opening reactions with secondary amines, where the methylated compounds did not react even under forcing conditions. In the light of the o-Ps lifetime measurements a possible explanation for this fact is that the methyl group sterically hinders the attack of the secondary amines. (C) 2001 Elsevier Science B.V. All rights reserved.